1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly, to an optical transmission system transmitting a multi-wavelength light by using Raman amplification.
2. Description of the Related Art
Conventionally, in a long-haul optical transmission system, 3-R processes (Re-timing, Re-shaping, and Re-generating) are executed in the state where an optical signal is once converted into an electric signal in each repeater, and the electric signal is reconverted into an optical signal, which is then transmitted to the next repeater. Currently, however, an optical amplifier that amplifies an optical signal without converting it into an electric signal is put into practical use, and a transmission system that adopts an optical amplifier as a linear repeater is under study. The repeater accompanied by the above described opt-electric conversion is replaced by an optical amplifier, so that the number of components configuring each repeater is significantly reduced. As a result, higher reliability and cost reduction promise to be achieved.
In the meantime, the amount of information transmitted via a network increases with the expansion of the Internet, and techniques for increasing the capacity of a transmission system are earnestly studied. A wavelength division multiplexing (WDM) optical transmission system is receiving attention as one method for increasing the capacity of a transmission system. The WDM optical transmission system is one way of multiplexing a plurality of signals by using a plurality of carriers having different wavelengths. With this system, the amount of information that can be transmitted over one optical fiber dramatically increases.
FIG. 1 shows the configuration of a general optical transmission system. In this system, a multi-wavelength light is transmitted from an optical transmitter 100 to an optical receiver 200. Namely, the optical transmitter 100 generates a multi-wavelength light by multiplexing signal lights having different wavelengths, and transmits the multi-wavelength light to a transmission line. In the meantime, the optical receiver 200 detects the respective signals by demultiplexing the received multi-wavelength light into the respective wavelengths. Here, the transmission line is an optical fiber, and optical amplifiers are spaced at predetermined intervals.
Each of the optical amplifiers is an erbium-doped fiber amplifier (EDFA). Here, the gain wavelength band of a general EDFA is a 1.55 xcexcm band, whereas that of a gain shift EDFA (GS-EDFA) is a 1.58 xcexcm band. The widths of these bands are approximately 30 nm, respectively. Accordingly, if EDFAs are spaced on a transmission line of a WDM optical transmission system, signal lights are transmitted by using carriers within these gain wavelength bands.
To increase the capacity of a transmission system, it is effective to increase the number of wavelengths to be multiplexed. One effective way of increasing the number of wavelengths to be multiplexed is to widen a gain wavelength band. In recent years, a Raman amplifier using Raman scattering has received attention as an optical amplification method that secures a wider gain wavelength band in comparison with an EDFA.
With Raman amplification, a gain is obtained on a side of a wavelength longer than that of a pump light by providing the pump light to an optical fiber. For example, in a 1.55 xcexcm band, a gain is obtained on a side of a wavelength that is longer by approximately 100 nm than the wavelength of a pump light as shown in FIG. 2A. This amount of shift is 13.2 Tera Hz when converted into a frequency. Additionally, the Raman amplifier can amplify an arbitrary wavelength if only a corresponding pump light can be prepared.
The Raman amplifier is implemented by using the above described nature. To obtain a wide gain wavelength band, a plurality of pump lights having different central frequencies are used as shown in FIG. 2B. This method is recited, for example, by Y. Emori, et al., xe2x80x9c100 nm bandwidth flat gain Raman amplifiers pumped and gain-equalized by 12-wavelength channel WDM high power laser diodesxe2x80x9d, OFC ""99 PD19, 1999xe2x80x9d. As described above, a wider gain wavelength band can be obtained by using a plurality of pump lights.
FIG. 3 shows the configuration of a WDM optical transmission system using Raman amplification. A pump light for Raman amplification is usually provided to a transmission line optical fiber so that it is transmitted to a direction reverse to a signal light. At this time, if a plurality of pump lights are used as shown in FIG. 2B, pump lights output from a plurality of light sources having different oscillation frequencies are provided to the transmission line optical fiber by a wavelength coupler (wavelength multiplexer) or the like.
For a long-haul optical transmission system in which a plurality of Raman amplifiers are required on a transmission line, challenges to be technically improved remain left. Specifically, it is desired that the powers of signal lights included in a multi-wavelength light are equalized, namely, the gain wavelength characteristics for a multi-wavelength light are desired to be flattened. This problem must also be considered in the case where the characteristic of a transmission line is changed due to repair work or deterioration with time.
The present invention aims at improving the transmission characteristic of an optical transmission system in which a plurality of Raman amplifiers are positioned on an optical transmission line, and more particularly, at improving the transmission characteristic of an optical transmission system in which each of Raman amplifiers uses a plurality of pump lights.
An optical transmission system according to the present invention is configured so that one or a plurality of Raman amplifiers are arranged on an optical transmission line. Each of the Raman amplifiers comprises: a plurality of pump light sources for producing pump lights for Raman amplification; and a multiplexer for multiplexing the pump lights produced by the plurality of pump light sources and for providing the multiplexed pump light to the transmission line, on which a gain equalizer is arranged.
In this system, a gain equalizer is arranged on an optical transmission line, whereby a multi-wavelength light is transmitted by being equalized even when an optical signal is amplified with a plurality of Raman amplifiers.
An optical transmission system according to another feature of the present invention is configured so that a plurality of Raman amplifiers are arranged on an optical transmission line, and each of the plurality of Raman amplifiers uses a plurality of pump lights. If the power of a pump light having a first wavelength among the plurality of pump lights drops to a predetermined level or lower in a first Raman amplifier among the plurality of Raman amplifiers, the power of the pump light having the first wavelength or a wavelength that is substantially the same as the first wavelength is raised in one or some of the plurality of Raman amplifiers other than the first Raman amplifier.
If the power of the pump light having the first wavelength drops in this system, the Raman gain of the wavelength region corresponding to the first wavelength decreases. Accordingly, if the power of the pump light having the first wavelength or a wavelength that is substantially the same as the first wavelength is raised in another or other amplifiers, the Raman gain is compensated. At this time, if the amount of rise in the pump light power is shared by a plurality of Raman amplifiers, a load can be prevented from being concentrated on a particular pump light source.
An optical transmission system according to a further feature is configured so that the power of a pump light having a wavelength adjacent to a first wavelength is raised in a first Raman amplifier or another or other Raman amplifiers, if the power of the pump light having the first wavelength among a plurality of pump lights drops to a predetermined level or lower in the first Raman amplifier among a plurality of Raman amplifiers.
With this system, the power of the pump light having a wavelength adjacent to the first wavelength is raised when the power of the pump light having the first wavelength drops. As a result, a decrease in a Raman gain, which is caused by the drop in the power of the pump light having the first wavelength, is compensated.
An optical transmission system according to a still further feature of the present invention is configured so that a plurality of Raman amplifiers are positioned on an optical transmission line, and each of the Raman amplifiers uses a plurality of pump lights. Each of the Raman amplifiers comprises a multiplexer for multiplexing the plurality of pump lights and for providing the multiplexed pump light to the optical transmission line. A plurality of multiplexers correspondingly provided for the plurality of Raman amplifiers are selected and arranged so that an average of the characteristics of the plurality of multiplexers results in a predetermined characteristic.
In this system, an average of the characteristics of the plurality of multiplexers for multiplexing a plurality of pump lights becomes a predetermined characteristic. Therefore, a desired gain wavelength characteristic can be obtained as an entire system even if the characteristics of the multiplexers vary.